1. Field of the Invention
The present invention relates generally to the field of optical isolators and more particularly to the cost, size reduction, and improved manufacturability of optical isolators.
2. Brief Description of Related Art
Typical optical isolators include a pair of collimating elements, such as GRIN (graduated index) lenses at the input and output ports of the device, and a core assembly located between the pair of collimating elements. The core assembly is made up of a pair of optically birefringent devices disposed on either side of an optically active device, such as a Faraday rotator.
FIG. 1 shows the structure of a typical optical isolator 8. Collimating elements 10 and 12 include metal collimator holders 25, 45, in which quartz collimator aligners 23 and 44 are positioned. Glass ferrules 22, 42 for holding optical fibers 21, 41, respectively and GRIN (graduated index of refraction) lenses 24, 43 are disposed within the quartz aligners 23, 44. GRIN lenses 24, 43 act to collimate the light rays entering the device and to focus any light rays leaving the device.
The core assembly 14 includes a cylindrical permanent magnet 35, birefringent wedges 31, 34 and a rotator 33, which is commonly made of yttrium iron garnet (YIG). The cylindrical permanent magnetic 35 generates a magnetic field for YIG rotator 33. Birefringent wedges 31 and 34 separate incident light into two orthogonal rays that are parallel and perpendicular to the optical axis of each wedge and wedge 34 has its optical axis rotated relative to the optical axis of wedge 31. The rotator 33 and birefringent wedges 31, 34 are held in place within the magnetic cylinder 35 by rotator holder 32.
There are two directions of operation, the forward direction in which light from fiber 21 enters the device and the reverse direction in which light from fiber 41 enters the device or reflections from light propagating in a forward direction enters the device.
Referring to FIG. 2A, in the forward direction of operation, light 60 from fiber 21, diagram (a), enters the GRIN lens 24 and is collimated onto birefringent wedge 31. The wedge 31 has two axes, a fast axis (minimum index of refraction nF) and a slow axis (maximum index of refraction nS) that are orthogonal to each other. To simplify the discussion, assume the fast axis is the horizontal and the slow axis is vertical, as shown. (In an actual implementation, the fast axis and slow axis can have a different angle with respect to the horizontal and vertical axes respectively.) Thus, regardless of the polarization of the input light, wedge 31 causes light emerging from the wedge to have a fast-axis component 64 and a slow-axis component 62, each component being refracted differently by the wedge 31, as shown in diagram (b) of FIG. 2A. This light is then processed by the optical rotator 33, which rotates the plane of polarization of both components in space by some angle xcex1, which depends on the thickness of the rotator 33. This is shown in diagram (c) of FIG. 2A. A typical rotation angle is 45 degrees. The spatially rotated components then impinge on wedge 34, which has its fast and slow axes rotated by an angle xcex2. If xcex2 is the same angle as xcex1, the fast component of the light beam is aligned with the fast axis of the wedge 34 and the slow component is aligned with the slow axis of the wedge 34. Because of this alignment, the light is refracted through wedge 34 without loss (ideally) to produce a collimated beam, as shown in diagram (d), that is focused by the GRIN lens 43 and accepted into the aperture of the fiber 41.
Referring to FIG. 2B, in the reverse direction of operation, light of arbitrary polarization 66 from fiber 41 or reflected light from a forward traveling wave enters the device and is collimated by GRIN lens 43 so that substantially parallel rays impinge upon wedge 34. Wedge 34, like wedge 31, has a fast axis and a slow axis, the axes rotated by the angle xcex2, as described above. Light passing in the reverse direction through wedge 34 now has fast component 70 and slow component 68 in diagram (b) of FIG. 2B, orthogonal to each other and aligned with the rotated axes of wedge 34. Next, the light beam passes, in the reverse direction, through the optical rotator 33, which, being a non-reciprocal device, rotates the planes of polarization, shown in diagram (c), by an angle xcex1 in the same direction as the rotation in the forward direction of travel. This rotation causes the components, 68, 70 of the light to be aligned with the vertical and horizontal axes. Light from the rotator is next processed by wedge 31 which has a horizontal fast axis and a slow vertical axis. However, because of the initial alignment of wedge 34 and the rotation of optical rotator 33, the slow component 68 from wedge 34 is aligned with the fast axis of wedge 31 and the fast component 70 from wedge 34 is aligned with the slow axis of wedge 31. The light beam is refracted by the wedge 31, according to this alignment, causing a pair of divergent beams to emerge from the wedge, as shown in diagram (d) of FIG. 2B. The divergent beams cannot be focused onto the aperture of the optical fiber 21 and the reverse-direction light is thus blocked from entering the fiber 21.
Current optical isolators, such as the one in FIG. 1, have lengths in the range of 40 mm to 42 mm and outer diameters in the range of 5.3 to 5.5 mm. These dimensions result from an internal structure of the isolator and its packaging in order to meet the optical performance, reliability and manufacturability requirements placed on the isolator.
A measure of the optical performance of an optical isolator is the ratio of the insertion loss to the isolation, where the insertion loss is the reduction in intensity of the signal in the forward direction of propagation through the isolator and isolation is the reduction in intensity of the signal in the reverse direction through the isolator. Ideally, the manufacture of the isolator is such as to minimize the insertion loss and maximize the isolation. To achieve this goal, the internal structure of the isolator must allow fine alignment adjustments of the collimators. Alignment of the isolator of FIG. 1 is accomplished by quartz collimator aligners 23 and 44. These components increase the outer diameter of the isolator of FIG. 1.
Reliability is measured by the ability of the isolator to withstand certain environmental stresses such as temperature, humidity and vibration without a significant impact on the optical performance, i.e., the insertion loss and isolation ratio, of the isolator. In part, reliability is enhanced by an outer protective cover surrounding the holder 25 and cylindrical magnet 35.
Finally, the manufacturability of the isolator is gauged by the manufacturing yield of isolators with good to superior optical performance characteristics. High yields of high performance devices translates into lower costs than with poor yields of high performance devices. An optical isolator may, in theory, be capable of superior performance that is not achievable in practice because the manufacturing process steps adversely affect the theoretical performance. An example of this is the use of high temperature solders to hold the collimating elements of the isolator in place. These solders can have a permanent and serious effect on the performance of the isolator by affecting the alignment of the collimating elements. This causes in irreparable loss in the performance of the isolator.
There is currently a demand for smaller and lower cost optical components such as optical isolators to reduce the overall size and cost of equipment using such components. However, reducing the size of an optical isolator precludes the use of currently available structures to meet the above-mentioned optical performance, reliability and manufacturability requirements placed on these components.
Thus, there is a need for a smaller and lower cost optical isolator component that meets or exceeds the performance, reliability and manufacturability requirements of current components.
The present invention is directed towards the above need. An apparatus in accordance with the present invention includes a housing tube, a first and second collimator, each of which is affixed within the housing tube to receive and collimate light signals from an optical fiber connected to each collimator and an optically isolating core assembly which also resides within the housing tube between the two collimators. The core assembly is joined directly to one of the collimators while the other collimator is alignable to the core assembly. Each of the collimators and the core assembly are pre-aligned to minimize insertion loss. The core assembly includes a cylindrical permanent magnet within which reside a pair of birefringent wedges and an optical rotator disposed between and joined on either side to the wedges. Each collimator includes a glass ferrule for holding an optical fiber and a GRIN lens for collimating light received from the optical fiber. Each collimator is affixed to the housing tube by means of solder joints and the alignable collimator is adjustable for alignment by means of these solder joints.
A method in accordance with the present invention includes the steps of providing an aligned first collimator and second collimator and an aligned core assembly. The core assembly is then joined to the first collimator and aligned to the first collimator to minimize insertion loss Then the first and second collimators are affixed within a housing tube and the second collimator is adjusted to be in alignment with the core assembly.
An advantage of the present invention is that the optical isolator is smaller in length because of the construction of the core assembly and small in diameter because of the construction of the collimators.
Another advantage of the present invention is that the optical isolator has improved optical performance because each of the elements is pre-aligned before assembly into the housing tube with the only adjustment required being that of the second collimator to the core assembly.
Another advantage is improved reliability because each element is pre-aligned and there is only a single element that needs alignment at final assembly.
Another advantage is improved manufacturability because only elements that have low insertion loss are assembled into a final unit with only one adjustment being made at final assembly, that of aligning the second collimator to the core assembly. This guarantees an improved yield of high quality devices.